Expected Input Credential not present while invoking Rest API

I am trying to invoke 3rd party Rest API (ViSA Merchant Locator API's) and have certificate (.p12) added in the appery resources. I am getting the following error while testing the service.

"responseStatus": {
"code": "9124",
"severity": "ERROR",
"info": "",
"status": 400,
"message": "Expected input credential was not present"

Could you please let me know what I must be doing wrong?
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